This invention relates to circuit card enclosures and, more particularly, to such an enclosure wherein the top and bottom walls are formed with integral circuit card guides and heat dissipation apertures which satisfy regulatory requirements without requiring an additional top cover member.
Complex electronic systems typically include a rectilinear sheet metal enclosure for the circuit cards making up the electronic system. Usually, a backplane board is supported along a back wall of the enclosure and card guides are provided on the top and bottom walls of the enclosure so that the circuit cards making up the electronic system can be slid into the enclosure for mating engagement with connectors provided on the backplane board. Since electronic components generate heat which must be dissipated from the enclosure, the top and bottom walls of the enclosure are usually provided with openings to allow the heat to escape. To minimize costs, the card guides are often formed directly from the sheet metal making up the top and bottom walls of the enclosure. These card guides are typically formed as a large opening which is cut and folded to form guides flanking the opening. However, regulatory agencies, such as Underwriters Laboratory (UL), have strict requirements on the maximum allowable size of openings on the top of such an enclosure. In particular, paragraph 4.3.15 of UL Standard 1950 requires that openings directly over bare parts at hazardous voltage shall not exceed five millimeters in any dimension or not exceed one millimeter in width regardless of length. In the past, to satisfy such a requirement, an additional cover or shield was added to the enclosure. It would therefore be desirable to provide an enclosure design which meets the regulatory requirements concerning allowable sizes of openings without requiring an additional cover or shield.